Stereochemistry of radical reactions at chiral carbon atoms

Another approach to obtaining information about the geometric requirements of free radicals has been to examine bridgehead systems. It will be recalled that the small bicyclic rings strongly resist formation of carbonium ions at bridgehead carbon atoms because the skeletal geometry prevents attainment of the preferred planar 

Studies of chemical reactivity have revealed no particularly strong resistance to formation of bridgehead radicals. In an early study, the decarbonylation of bridgehead aldehydes was found to proceed without difficulty  

To summarize, structural studies of radicals and the stereochemistry of radical reactions both lead to the conclusion that radical centers are not rigid pyramids. Instead, they are either planar or shallow pyramids, and barriers to inversion of the pyramidal structures are low. Radicals also seem to be able to tolerate small geometric distortions associated with strained rings without great increase in their energy. 

There has also been study of the stereochemistry of vinyl free radicals. Radicals formed at trigonal centers can rapidly undergo equilibration with the geometric isomer, giving rise to the same product mixture from isomeric cis and trans starting materials  

General reviews of structure and reactivity of vinyl radicals W. G. Bentrude, Anna. Rev. Phys. Chem. 18,283 (1967) L. A. Singer, in Selective Organic Transformations, Vol. II, B. S. Thyagarajan (ed.), John Wiley, New York, 1972, p. 239 O. Simamura, Top. Stereochem. 4,1 (1969). 

---

Scheme 12.2. Stereochemistry of Radical Reactions at Chiral Carbon Atoms...

Stereochemistry of radical reactions at chiral carbon atoms... 

---